Superchargers are known to increase engine output torque by increasing the intake air pressure. One well-known example of a supercharger is an exhaust turbocharger, hereinafter described as “a turbocharger.”
Turbochargers typically include a turbine wheel (“a turbine”) in an exhaust passage coupled by a shaft to a compressor wheel (“a compressor”). In operation, exhaust gas passing by the turbine spins a shaft which rotates the compressor and increases the intake pressure. The pressure increase in the intake air is sometimes referred to as charging the air, and the compressor output may be called charge air.
Although a turbocharger can obtain high charging pressure efficiently, it cannot necessarily increase engine torque in a broad engine speed range that includes both low and high engine speeds. Generally, a small-sized turbocharger increases the engine torque in a low engine speed range and a large-sized turbocharger increases the engine torque in a high engine speed range. Therefore, it may be necessary to select a turbocharger based on an engine torque characteristic when a turbocharger is provided with an engine.
However, many engine systems need to increase engine torque over a large engine speed range from low engine speed to high engine speed. To address this need, a variety of supercharging devices may be employed, such as a system with two turbochargers, one of which is used in a low engine speed range and the other of which is used in a high engine speed range; a system including an electric supercharger for use in a low engine speed range and a turbocharger for use in a high engine speed range; and a system including a turbocharger including a movable flap which may be adjusted according to the engine speed (a “variable-geometry turbocharger” as described in Japanese Unexamined Patent Application Publication No. 1997-112285).
Where a variable-geometry turbocharger is employed, it may be desirable to select a large-sized turbocharger as a base turbocharger.
In such a system, engine torque may be increased in a high engine speed range due to innate characteristics of the large-sized turbocharger. In a low engine speed range, engine torque may be increased by reducing the flap opening, which increases the exhaust flow rate and subsequently increases the turbine driving force. Accordingly, this system can increase engine torque over a large engine speed range.
However, a system including a variable-geometry turbocharger typically has a complicated structure. Further, the large size of these devices may crowd the engine room. Therefore, there is a need for an engine system with a turbocharger capable of increasing engine torque in a low engine speed range while having a simple structure.
One aspect of the present invention includes an engine system with a turbocharger comprising: an exhaust manifold having plural independent exhaust passages, each of the exhaust passages being connected to an exhaust port of a corresponding engine cylinder; a collective part formed by gathering said independent exhaust passages in said exhaust manifold or on a downstream side of said exhaust manifold; an exhaust turbocharger connected to a downstream side of said collective part; a variable exhaust valve for changing each passage cross-sectional area of said independent exhaust passage at an upstream side of said collective part; and a controller for controlling said variable exhaust valve, wherein said controller is configured to perform independent exhaust throttle control for reducing a passage cross-sectional area of at least one of said independent exhaust passages below a maximum area with said variable exhaust valve, at least in a predetermined low engine speed range in a supercharging operation range of said engine.
This system overcomes at least some of the disadvantages of the above-discussed references. Specifically, an exhaust gas ejector effect can be generated by the independent exhaust passages and a variable exhaust valve. This ejector effect, also referred as “a jet-pump,” can extract a fluid with a vacuum generated by a driving fluid having a high flow velocity. In this specification, a vacuum operating on the same principle as this effect is hereinafter also called “the vacuum effect”.
In the embodiments described in this description, exhaust gas, specifically, blow-down gas that flows with high flow rate just after an exhaust valve opens, flows in an independent exhaust passage. The area of the exhaust gas flow is narrowed by the variable exhaust valve, which reduces the passage cross-sectional area of the independent exhaust passage, causing the flow rate of the exhaust gas to increase and causing the pressure of the exhaust gas to decrease. Under these conditions, the exhaust gas corresponds to the driving fluid of the above-described ejector.
The exhaust gas may also flow through a collective part, by which an exhaust passage may communicate with other exhaust passages. As a result, the exhaust gas, functioning as a driving fluid, may suck out, or extract, an exhaust gas in another exhaust passage by the vacuum effect. To enhance this vacuum effect, it is desired that the exhaust gas functioning as a driving fluid merge with extracted exhaust gas at an acute angle.
The ejector has three major benefits, as follows.
First, the ejector effect can increase the turbine flow of the turbocharger, which is the amount of exhaust gas supplied to a turbocharger. The exhaust gas, functioning as a driving fluid, merges with the extracted exhaust gas in the collective part and the merged gas is introduced into a turbine located downstream of the collective part, which results in increasing turbine flow by the amount of extracted exhaust gas in comparison with the case of no ejector effect. Accordingly, the ejector effect can increase turbine driving force, thereby increasing charging pressure.
Second, the ejector effect may facilitate exhaust gas scavenging. Exhaust gas extracted by the ejector may serve as scavenged exhaust gas, which may decrease the exhaust resistance. Further, air inspiration during the overlap period (a period during which both an intake and an exhaust valve are open, such as during a transition from an exhaust stroke to an intake stroke) is enhanced by scavenging. As a result, intake volume may be increased, thereby increasing engine torque. To enhance this effect, the blow-down timing of one engine cylinder, which corresponds to the timing just after exhaust valve opening for one engine cylinder, may be adjusted to coincide with an overlap period for another engine cylinder. However, it is noted that such an example requires that the exhaust passages for each cylinder must be independent of each other at the upstream side of the variable exhaust valve.
Third, the ejector effect may also enhance a dynamic pressure charge. The dynamic pressure charge increases the charging performance of a turbocharger by using an exhaust pulsation. The effect may be enhanced as the exhaust pulsation becomes larger. Reducing the exhaust passage volume is an effective approach to enlarge the exhaust pulsation. However, reducing the total volume of the exhaust manifold to reduce the exhaust passage volume may be constrained by the exhaust system layout.
In prior approaches that do not include an ejector effect, exhaust gas from one exhaust passage may flow between exhaust passages in the collective part. In other words, exhaust gas from one exhaust passage may flow back into another exhaust passage. Unlike those approaches, the approach provided by an embodiment described in this description includes an ejector effect utilizing exhaust gas from one exhaust passage as a driving fluid, extracting fluid from another exhaust passage. Thus, exhaust gas from one exhaust passage does not flow back to another exhaust passage. This reduces an exhaust passage volume in a dynamic pressure charge operation.
Thus, the addition of the ejector effect can enhance a dynamic pressure charge in systems where the total volume of the exhaust passages and the exhaust manifold volume are the same.
Further, in some embodiments of this invention, it is desirable to include a large-sized turbocharger. In such a system, engine torque is increased by using an innate characteristic of the large-sized turbocharger in a high engine speed range. In a low engine speed range, engine torque is increased via the above-described ejector effect. Accordingly, this system can increase engine torque over large engine speed range from a low engine speed to a high engine speed.
In an example embodiment, said predetermined low engine speed range is a range where an engine speed is lower than a predetermined engine speed at which a waste gate valve of said exhaust turbocharger begins to open, wherein said controller is further configured to decrease said passage cross-sectional area as the engine speed becomes lower during said independent exhaust throttle control.
This may result in an effective ejector effect throughout an engine operating range, which may have fewer negative effects associated with the above described references, as described below.
The increase in charging pressure caused by the ejector effect is more pronounced when the engine speed is lower than the speed at which a waste gate valve of the turbocharger starts to open (“the intercept point”).
When the engine speed is above the intercept point, the waste gate valve is opened to avoid excessive charging pressure. Thus, increasing the charging pressure with the ejector effect is unnecessary above the intercept point. Further, independent exhaust throttle control when the engine speed is above the intercept point may lead to an undesirable exhaust resistance.
Accordingly, in one embodiment the ejector effect is utilized when the engine speed is below the intercept point, providing an engine operating range in which independent exhaust throttle control is effective with few negative effects.
In a low engine speed range wherein the independent exhaust throttle control is performed, the demand for increased charging pressure grows as the engine speed is reduced. Further, the ejector effect grows as the exhaust passage cross-sectional area decreases. Therefore, reducing the exhaust passage cross-sectional area as the demand for increased charging pressure grows can magnify the charging pressure enhancement provided by the ejector effect.
In another example embodiment, the system further comprises an exhaust valve timing changing mechanism, wherein said controller is configured to retard an exhaust valve opening timing via said exhaust valve timing changing mechanism such that an exhaust valve opening timing at which said independent exhaust throttle control is performed is later than an exhaust valve opening timing at which said independent exhaust throttle control is not performed.
This may result in an effective ejector effect as described in detail below.
Under conditions wherein the independent exhaust throttle control is not performed, an exhaust valve starts to open early, before bottom dead center of the exhaust stroke. For example, the exhaust valve may open between 40 and 60 degrees of crank angle before bottom dead center. This may enhance scavenging while weakening the blow-down gas flow because the exhaust action starts when the piston is dropping. However, the weakened blow-down gas flow may be less effective as a driving fluid for the ejector effect, which may diminish the capabilities of the independent exhaust throttle control.
One approach to address this is to retard the exhaust valve opening timing. For example, the exhaust valve opening timing may be retarded to after bottom dead center of the exhaust stroke, which may inhibit the decrease in blow-down gas flow. Further, the piston begins to rise in the combustion chamber after the crank passes bottom dead center. This piston action, which may enhance the blow-down gas flow and the ejector effect, may be harnessed by delaying the opening of the exhaust valve until after the crank passes bottom dead center.
However, opening the exhaust valve after bottom dead center of the exhaust stroke may increase the exhaust resistance. Therefore, it may be desirable to open the exhaust valve at least just before bottom dead center of exhaust stroke, even when the exhaust valve opening is otherwise retarded.
In an example embodiment, the system further comprises a valve timing changing mechanism capable of changing an overlap period wherein both of an intake and an exhaust valve are opened by changing at least one of intake and exhaust valve opening or closing timing, wherein said controller is configured to enlarge said overlap period via said valve timing changing mechanism such that an overlap period at which said independent exhaust throttle control is performed is larger than an overlap period at which said independent exhaust throttle control is not performed.
This may result in an effective ejector effect as described in detail below.
As explained above, the independent exhaust throttle control of an embodiment of this invention may increase the intake air pressure, thereby increasing engine torque, caused, for example, by an enhancement in scavenging, an enhancement of an inspiration overlap period, etc. In this example embodiment, an ejector effect can be achieved by enlarging the overlap period via the valve timing changing mechanism when performing the independent exhaust throttle control.
Although enlarging the overlap period usually makes exhaust gas flow back toward the combustion chamber in response to a negative pressure condition caused by air intake, there is no backflow in this embodiment. Instead, the exhaust gas flows toward the downstream side of the exhaust passage in response to the ejector effect. In other words, in this example, the overlap period can be enlarged while reducing exhaust gas backflow.
Here, the above-described valve timing changing mechanism for enlarging the overlap period may be a device for advancing the intake valve opening timing, a device for retarding the exhaust valve closing timing, or a device for both of advancing intake valve opening timing and retarding exhaust valve closing timing. When a conventional variable valve timing (VVT) device that can move both of valve opening and closing timing backward and forward while maintaining the opening period is employed as the valve timing changing mechanism, and when this device is configured such that at least the exhaust valve closing timing is retarded, the exhaust valve opening timing is retarded automatically. In other words, this device is also used as the above-mentioned exhaust valve timing changing mechanism.
In an example embodiment, said controller is further configured to perform said independent exhaust throttle control in a naturally-aspirated range of said engine, and to enlarge said overlap period via said valve timing changing mechanism when said independent exhaust throttle control is performed, such that an overlap period at which said independent exhaust throttle control is performed in said naturally-aspirated range is larger than an overlap period at which said independent exhaust throttle control is not performed in said naturally-aspirated range.
This can enhance the response of the enlarged overlap period when switching from a naturally-aspirated range to a super-charging range as described below.
Generally, an engine with a valve timing changing mechanism is set such that the overlap period is enlarged at higher engine loads and/or at higher engine speeds. Therefore, an overlap period in a naturally-aspirated range that has a lower engine load range may be shorter than an overlap period in a super-charging range that has a higher engine load range, even at the same engine speed.
Thus, if the engine load is increased, for example by an acceleration demand, and the engine operating range quickly switches from a lower engine load range to a higher engine load range, a valve timing changing mechanism may correspondingly quickly increase the overlap period. When there is a large increase in the rate of change for the overlap period, a response lag between the valve timing change and the overlap period change may result.
Under some conditions where the amount of overlap increase is large, the response lag in changing valve timing may be significant. For example, the response lag may be significant in a supercharging operation range of the engine in which independent exhaust throttle control is used at the same time that the overlap period is increased.
Thus, according to one embodiment, independent exhaust throttle control is performed and the overlap period is enlarged in a naturally-aspirated range of said engine, reducing the response lag of changing valve timing when switching from a naturally-aspirated range to a super-charging range.
In an another example embodiment, the system further comprises a valve timing changing mechanism capable of changing at least one of an intake and an exhaust valve opening or closing timing and a fuel injector capable of changing an injected fuel amount to change an air-fuel ratio of a mixture provided in a combustion chamber of said engine, wherein said controller is further configured to perform an afterburning mode when said independent exhaust throttle control is performed, in which said controller controls said fuel injector to increase the air-fuel ratio in said combustion chamber to a richer than stoichiometric value, and controls said valve timing changing mechanism to enlarge an overlap period wherein both of the intake and the exhaust valve are opened to or above a predetermined range such that an unburned fuel is exhausted from said engine and combusted at an upstream side of said exhaust turbocharger.
In this embodiment, so-called “afterburning” occurs by creating conditions such that unburned fuel combusts upstream of the exhaust turbocharger when performing independent exhaust throttle control.
Afterburning refers to a phenomenon wherein unburned fuel is exhausted during the overlap period of the intake and the exhaust valves. Afterburning may result from a combination of setting the air-fuel ratio of mixture richer than stoichiometric value (but still within the burnable range) and mixing the exhausted unburned fuel with ambient air under the high pressure condition due to the increased blow-down peak caused by the ejector effect.
Engine output can be improved by enlarging the overlap period while performing independent exhaust throttle control. For example, increasing the overlap period can increase the scavenged burned gas, which may lead to improved charging efficiency, which may lead to improved engine output. Once the overlap period reaches a threshold, the engine output may remain constant despite continued enlargements to the overlap period. This change in output response may occur when the charge air remaining in the combustion chamber is no longer proportional to the amount of air passing through the combustion chamber in response to valve overlap.
However, the inventors herein have recognized that, if the overlap period continues to be increased after the engine output reaches the threshold described above, unburned fuel will be exhausted into the exhaust passage. This unburned fuel may combust at the upstream side of said exhaust turbocharger, which results in the afterburning phenomenon.
Afterburning can improve charging performance because it may increase the exhaust pressure, thereby increasing the charging pressure. Therefore, this embodiment can improve charging performance in a low engine speed range even when a large-sized turbocharger is adopted.
In one example embodiment, the system further comprises an electric supercharger provided in an air intake passage of said engine, wherein said controller is configured to activate said electric supercharger when said overlap period is within torque stationary range in said afterburning mode.
This embodiment can compensate for diminished engine output by operating an electric supercharger when intake and exhaust valves are operated with an overlap period and where afterburning does not occur.
As described above, when the overlap period is enlarged while performing independent exhaust throttle control, there may be a torque stationary range between an overlap amount for improving scavenging performance and an overlap amount for improving charging performance by afterburning phenomenon. Scavenging performance may be saturated and afterburning may occur within this range.
Therefore, in an embodiment wherein the intake and the exhaust valves are operated within the above-described range, an electric supercharger may sustain the charging performance to maintain engine output within a broad operating range. Further, if the electric supercharger is only operated in the above-described range, supercharger operation can be reduced.
In one example embodiment, said controller is configured to adjust said overlap period to more than a ninety-degree crank angle in said afterburning mode.
This embodiment may trigger the afterburning phenomenon with high probability under engine operating conditions where an afterburning mode should be performed, which may result in improved charging performance and engine output.
The above advantages and other advantages and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.